Re-enable pre-read experiment as a finch field trial.

chrome/app/client_util.cc

Index: chrome/app/client_util.cc
diff --git a/chrome/app/client_util.cc b/chrome/app/client_util.cc
index 4aa2336e63ff187ab86e9609425fc8a19561041a..08c58192e581dd82604d789532f80dc54c4c8dcb 100644
--- a/chrome/app/client_util.cc
+++ b/chrome/app/client_util.cc
@@ -11,6 +11,8 @@
 #include "base/file_version_info.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/rand_util.h"  // For PreRead experiment.
+#include "base/sha1.h"  // For PreRead experiment.
 #include "base/strings/string16.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
@@ -37,6 +39,112 @@ typedef int (*DLL_MAIN)(HINSTANCE, sandbox::SandboxInterfaceInfo*);
 
 typedef void (*RelaunchChromeBrowserWithNewCommandLineIfNeededFunc)();
 
+// Returns true if the build date for this module precedes the expiry date
+// for the pre-read experiment.
+bool PreReadExperimentIsActive() {
+  const int kPreReadExpiryYear = 2014;
+  const int kPreReadExpiryMonth = 7;
+  const int kPreReadExpiryDay = 1;
+  const char kBuildTimeStr[] = __DATE__ " " __TIME__;
+
+  // Get the timestamp of the build.
+  base::Time build_time;
+  bool result = base::Time::FromString(kBuildTimeStr, &build_time);
+  DCHECK(result);
+
+  // Get the timestamp at which the experiment expires.
+  base::Time::Exploded exploded = {0};
+  exploded.year = kPreReadExpiryYear;
+  exploded.month = kPreReadExpiryMonth;
+  exploded.day_of_month = kPreReadExpiryDay;
+  base::Time expiration_time = base::Time::FromLocalExploded(exploded);
+
+  // Return true if the build time predates the expiration time..
+  return build_time < expiration_time;
+}
+
+// Get random unit values, i.e., in the range (0, 1), denoting a die-toss for
+// being in an experiment population and experimental group thereof.
+void GetPreReadPopulationAndGroup(double* population, double* group) {
+  // By default we use the metrics id for the user as stable pseudo-random
+  // input to a hash.
+  base::string16 metrics_id;
+  GoogleUpdateSettings::GetMetricsId(&metrics_id);
+
+  // If this user has not metrics id, we fall back to a purely random value
+  // per browser session.
+  const size_t kLength = 16;
+  std::string random_value(metrics_id.empty() ? base::RandBytesAsString(kLength)
+                                              : base::WideToUTF8(metrics_id));
+
+  // To interpret the value as a random number we hash it and read the first 8
+  // bytes of the hash as a unit-interval representing a die-toss for being in
+  // the experiment population and the second 8 bytes as a die-toss for being
+  // in various experiment groups.
+  unsigned char sha1_hash[base::kSHA1Length];
+  base::SHA1HashBytes(
+      reinterpret_cast<const unsigned char*>(random_value.c_str()),
+      random_value.size() * sizeof(random_value[0]),
+      sha1_hash);
+  COMPILE_ASSERT(2 * sizeof(uint64) < sizeof(sha1_hash), need_more_data);
+  const uint64* random_bits = reinterpret_cast<uint64*>(&sha1_hash[0]);
+
+  // Convert the bits into unit-intervals and return.
+  *population = base::BitsToOpenEndedUnitInterval(random_bits[0]);
+  *group = base::BitsToOpenEndedUnitInterval(random_bits[1]);
+}
+
+// Gets the amount of pre-read to use as well as the experiment group in which
+// the user falls.
+size_t InitPreReadPercentage() {
+  // By default use the old behaviour: read 100%.
+  const int kDefaultPercentage = 100;
+  const char kDefaultFormatStr[] = "%d-pct-default";
+  const char kControlFormatStr[] = "%d-pct-control";
+  const char kGroupFormatStr[] = "%d-pct";
+
+  COMPILE_ASSERT(kDefaultPercentage <= 100, default_percentage_too_large);
+  COMPILE_ASSERT(kDefaultPercentage % 5 == 0, default_percentage_not_mult_5);
+
+  // Roll the dice to determine if this user is in the experiment and if so,
+  // in which experimental group.
+  double population = 0.0;
+  double group = 0.0;
+  GetPreReadPopulationAndGroup(&population, &group);
+
+  // We limit experiment populations to 1% of the Stable and 10% of each of
+  // the other channels.
+  const string16 channel(GoogleUpdateSettings::GetChromeChannel(
+      GoogleUpdateSettings::IsSystemInstall()));
+  double threshold = (channel == installer::kChromeChannelStable) ? 0.01 : 0.10;
+
+  // If the experiment has expired use the default pre-read level. Otherwise,
+  // those not in the experiment population also use the default pre-read level.
+  size_t value = kDefaultPercentage;
+  const char* format_str = kDefaultFormatStr;
+  if (PreReadExperimentIsActive() && (population <= threshold)) {
+    // We divide the experiment population into groups pre-reading at 5 percent
+    // increments in the range [0, 100].
+    value = static_cast<size_t>(group * 21.0) * 5;
+    DCHECK_LE(value, 100u);
+    DCHECK_EQ(0u, value % 5);
+    format_str =
+        (value == kDefaultPercentage) ? kControlFormatStr : kGroupFormatStr;
+  }
+
+  // Generate the group name corresponding to this percentage value.
+  std::string group_name;
+  base::SStringPrintf(&group_name, format_str, value);
+
+  // Persist the group name to the environment so that it can be used for
+  // reporting.
+  scoped_ptr<base::Environment> env(base::Environment::Create());
+  env->SetVar(chrome::kPreReadEnvironmentVariable, group_name);
+
+  // Return the percentage value to be used.
+  return value;
+}
+
 // Expects that |dir| has a trailing backslash. |dir| is modified so it
 // contains the full path that was tried. Caller must check for the return
 // value not being null to determine if this path contains a valid dll.
@@ -52,19 +160,12 @@ HMODULE LoadChromeWithDirectory(string16* dir) {
   dir->append(dll_name);
 
 #if !defined(WIN_DISABLE_PREREAD)
-  // On Win7 with Syzygy, pre-read is a win. There've very little difference
-  // between 25% and 100%. For cold starts, with or without prefetch 25%
-  // performs slightly better than 100%. On XP, pre-read is generally a
-  // performance loss.
+  // We pre-read the binary to warm the memory caches (fewer hard faults to
+  // page parts of the binary in).
   if (!cmd_line.HasSwitch(switches::kProcessType)) {
     const size_t kStepSize = 1024 * 1024;
-    // Pre-read 100% of the binary. This was the fallback behaviour for an
-    // expired pre-read experiment as well as the standard behaviour before
-    // the pre-read experiment. Performance regressed when the experiment
-    // (and fallback to 100%) was removed. See http://crbug/245435
-    // TODO(rogerm): Revive the pre-read experiment under finch.
-    uint8 percent = 100;
-    ImagePreReader::PartialPreReadImage(dir->c_str(), percent, kStepSize);
+    size_t percentage = InitPreReadPercentage();
+    ImagePreReader::PartialPreReadImage(dir->c_str(), percentage, kStepSize);
   }
 #endif